Ultrasensitive magnetic sensor with magneto-optically active material with high verdet constant

ABSTRACT

A high-resolution sensor of magnetic field sensor system and materials for use in such a system are described. The sensor systems measure a magnetic field using inorganic and/or organic magneto-optically active materials, e.g. polymer material and have an interferometer based on Faraday rotation. The polymer material is preferably in the form of a film. The polymer material has an optical property that is sensitive to the magnetic field, e.g. the Faraday rotation effect. The present invention also provides a sensor head structure comprising the above polymer material. The sensor head may be designed for use with an optical fiber or with mirrors. In particular the present invention provides a fiber Sagnac interferometer to measure the rotation of polarized plane of light. The present invention provides a fiber or mirror based Sagnac interferometer with passive phase bias applied to magnetic field sensing. This invention has the following three major aspects each being an embodiment of the present invention: 1. Sensing material: a conjugate polymer such as polythiophene and/or a polymer containing superparamagnetic nanoparticles that exhibits a giant Faraday rotation. 2. Magnetic field probe that incorporates the above polymer or polymer/nanoparticle composite, e.g. in the form of a film. 3. Sensor: The sensor combines an optical fiber-based Sagnac interferometer and the above magnetic field probe.

This is a 371 national phase application of PCT/US2009/031313 filed 16Jan. 2009, the content of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a high-resolution sensor ofelectromagnetic field as well as to methods of constructing the same. Inparticular the present invention relates to a high-resolution sensor ofmagnetic field as well as to methods of constructing the same, e.g.,that can also be used for the measurement of electric current and alsoelectric fields.

BACKGROUND OF THE INVENTION

The measurement of very small magnetic fields is very important forbiomedical research, e.g., for MCG (magneto-Cardiogram, fields smallerthan 1 nanoTesla) and for MEG (Magneto-Encephalogram, fields smallerthan 10 picoTesla). Currently these magnetic fields are measured withSQUID devices—Superconducting Quantum Interference Devices (SQUID) arevery sensitive magnetometers used to measure extremely small magneticfields (up to femtoTesla). These SQUID devices are only working atcryogenic temperatures and are extremely expensive, prohibiting theiruse in current medical practice.

The working principle Faraday rotation is shown schematically in FIG. 1.Faraday rotation is the rotation of the polarization plane of linearlypolarized light due to magnetic field induced birefringence by amagnetic field parallel with the direction of light propagation. Faradayrotation has been known for more than hundred years and is commonly usedin optical isolators. The rotation of the polarization plane (θ) isgiven by the product of the magnitude of the magnetic field (B), thesample length (L) and the Verdet constant (V): θ=VBL. The Verdetconstant, which quantifies the Faraday effect, is for all materialsdifferent from zero and strongly wavelength dependent.

The principles of operation of a Sagnac interferometer are described in“The optical fibre Sagnac interferometer: an overview of its principlesand applications”, B. Colshaw, Inst. of Physics Publishing, Meas. Sci.Technol. Vol. 17, (2006) R1-R16.

The article by Jing Xia, et al. entitled “Modified Sagnac interferometerfor high-sensitivity magneto-optic measurements at cryogenictemperatures”, Appl. Phys. Letters, vol. 89, 06258 (2006) describes azero-area Sagnac interferometer loop for measuring the magneto-opticKerr effect at cryogenic temperatures. A beam of light from a broadbandsource is routed by a fiber to a polarizer and from there to a half-waveplate and an electro-optic modulator and from there to a fiber. Thelight emitted from the end of the fiber is reflected off the samplewhich is maintained at cryogenic temperatures.

The most common way of constructing a fiber Sagnac interferometer is byusing the lock-in detection based modulator-demodulator scheme. Thedetection bandwidth of the lock-in scheme is limited by the modulationfrequency, and this scheme requires costly optical and electroniccomponents such as fiber-pigtailed optical phase modulator and a lock-inamplifier.

MEMS-based magnetometers are known that are used to measure relativelystrong magnetic fields above microTesla level, and ultra-sensitivespectroscopy-based devices and SQUIDS. The MEMS magnetometers areextremely compact and inexpensive, but their sensitivity is limited.They are not suitable for biomedical applications. In addition, MEMSmagnetometers are sensitive to effects of electromagnetic interference.

Atomic magnetometers (such as optically pumped magnetometers) havesensitivity of up to a picoTesla, but they are very fragile andexpensive. They utilize several narrow-line, stabilized single-frequencylaser sources for interrogation of atomic transitions in gas cells.SQUIDs that presently have the highest sensitivity of all magnetometertypes are very expensive and impractical as they have to be operated atcryogenic temperatures.

SUMMARY OF THE INVENTION

The present invention provides a sensor system, an inorganic and/ororganic magneto-optically active probe, inorganic and/or organicmagneto-optically active materials, an inorganic and/or organicmagneto-optically active film and methods of making the same as definedin the attached claims. These are described in more detail in thedescription with reference to the drawings.

The present invention provides a magnetic sensor system and materialsfor use in such a system. Sensor systems according to the presentinvention measure a magnetic field using inorganic and/or organicmagneto-optically active materials, e.g., polymer material and have aninterferometer based on Faraday rotation. The polymer material ispreferably in the form of a film. The polymer material has an opticalproperty that is sensitive to the magnetic field, e.g., the Faradayrotation effect.

The present invention also provides a sensor head structure comprisingthe above polymer material. The sensor head may be designed for use withan optical fiber or with mirrors. The present invention provides amagnetic sensor probe comprising:

an inlet for coupling in light,

an outlet for coupling out light,

an inorganic and/or organic magneto-optically active film which isarranged to transmit the light received by the magnetic sensor probe,and

optical components to direct the light from the inlet, through theinorganic and/or organic magneto-optically active film to the outlet.

In particular the present invention provides a fiber Sagnacinterferometer to measure the rotation of polarized plane of light. Thepresent invention provides a fiber or mirror based Sagnac interferometerwith passive phase bias applied to magnetic field sensing.

This invention has the following three major aspects each being anembodiment of the present invention:

1. Sensing material: a conjugate polymer such as polythiophene and/or apolymer containing superparamagnetic nanoparticles that exhibits a giantFaraday rotation.

2. Magnetic field probe that incorporates the above polymer orpolymer/nanoparticle composite, e.g. in the form of a film.

3. Sensor: The sensor combines an optical fiber-based Sagnacinterferometer and the above magnetic field probe.

In the sensor, polarized light is guided to the polymer-based probe. Thepolymer-based probe may be a polymer magnetic-field probe. The polarizedlight may be guided by an optical fiber or by other optical means, e.g.,by use of at least one mirror. After passing through the polymer, thelight is guided to a detector. The light may be incident at 90° to thepolymer film or it may be incident at an angle thereto. The light fromthe polymer may be guided by the same or a different optical fiber or byother optical means, e.g., by use of at least one mirror. A preferredembodiment of the sensor includes an interferometer of which an in-lineSagnac-type interferometer is one suitable type. In a preferredembodiment, the light is delivered to the polymer probe by the samefiber that carries light back to the detector. The magnetic field to bemeasured causes the polarization rotation of light in the magnetic-fieldprobe, which is converted into the intensity variation in the detector,by the interferometer, e.g., Sagnac interferometer.

The sensitivity of a sensor according to the present invention can be inthe nanoTesla range. With a probe based on an improved polymers orpolymer/nanoparticle composites, as described with respect to thisinvention, the resolution can reach the femtoTesla range.

Accordingly, the present invention provides for an ultrasensitivemagnetometer for measurement of magnetic field strength, e.g., at andbelow the nanoTesla range and at or below the femtoTesla range. Thissensor can be used for robotic and monitoring applications in varioustechnology and biomedical fields. A principle of operation of the deviceis based on the Giant Faraday Effect in films with a high Verdetconstant. In embodiments of this invention, conjugated polymers andpolymer/nanoparticle blends are used as a material with high Verdetconstant. The films may be made, for example, of conjugated polymers orcomposites of polymers and superparamagnetic nanoparticles. The polymersneed not be limited to only polythiophene-based polymers. For example,the conjugated polymer can be poly(acetylene), poly(pyrrole),poly(aniline), poly(fluorene), poly(thiophene), polytetrathiafulvalenes,polynaphthalenes, poly(p-phenylene sulphide), poly(para-phenylenevinylene), polyalkoxythiophene.

Other polymer films incorporating superparamagnetic nanoparticles ofmagnetic metal-oxide composites can also be utilized. The amount ofsuperparamagnetic particles in the polymer can range from 0.1 to 50 wt%, e.g. 1-10 wt %. The polymer forming the matrix can be any suitabletransparent polymer such as PMMA, polyethylene, polypropylene,polycarbonate, ABS, PVC, PVA, polyimide, polystyrene, etc., or aconjugate polymer as described with reference to this invention. Inaddition, polymers containing superparamagnetic nanoparticles can bepoled by an external magnetic field to enhance the Verdet constant andreduce optical loss. The amount of Faraday rotation is proportional tothe magnetic field strength and is measured using an interferometer. Inone embodiment an in-line fiber Sagnac interferometer is used. Thisinterferometer may comprise, for example, an LED, or a superluminescentdiode or laser diode as a light source. In a preferred embodiment of thesensor, the shot-noise limited resolution of the sensor is in the 1pT/sqrt(Hz) range. The sensitivity limit can be improved by implementinga magnetic field concentrator, a well-known technique in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically the effect of Faraday rotation.

FIG. 2 shows schematically an embodiment of a sensor in accordance withthe present invention.

FIG. 3 shows results obtained with sensing materials according toembodiments of the present invention.

FIG. 4 shows schematically a further embodiment of a sensor inaccordance with the present invention.

FIG. 5 a shows schematically an embodiment of a sensor probe inaccordance with the present invention and FIG. 5 b shows a sensor withwhich this probe can be used.

FIG. 6 shows schematically a further embodiment of a sensor probe inaccordance with the present invention.

FIG. 7 shows results of measuring the earth's magnetic field using anembodiment of a sensor in accordance with the present invention.

FIG. 8 illustrates the Faraday rotation for a fiber Sagnacinterferometer according to the present invention.

DETAILED DESCRIPTION

The present invention provides methods, devices and materials for themeasurement of very small magnetic fields, e.g., for use in biomedicalresearch, e.g., for MCG (magneto-Cardiogram), e.g., to measure magneticfields smaller than 1 nanoTesla, and/or for MEG (Magneto-Encephalogram),e.g., to measure fields smaller than 10 picoTesla. The present inventionprovides very sensitive magnetometers and methods of use to measureextremely small magnetic fields, e.g., down to nanoTesla or down tofemtoTesla. The devices according to the present invention can work atroom temperature, e.g., below 200° C. They may also be operated at lowertemperatures, e.g., at cryogenic temperatures.

Embodiments of the present invention have advantage that they allow themeasurement of extremely weak magnetic fields at room temperature in avery simple way. The magnetic sensor, based upon new materials, is easyto manufacture, low cost and can be integrated in a complete system tomeasure biomagnetism. Since the measurement is based upon the Faradayrotation effect, an optical effect, it is not sensitive to electricinterference and would compensate for the magnetic field of the earth, amajor problem in measurement of very small magnetic fields.

In accordance with one aspect of the present invention, a structure of afiber-tip sensor is provided. The structure can be of the one fiber typeor two-fiber type which represent two preferred embodiments of themagnetic-field probe.

FIG. 2 shows schematically a one fiber embodiment of a sensor inaccordance with the present invention. FIG. 2 shows a particularembodiment of the present invention in the form of an in-line fiberSagnac interferometer. As a light source 2 typically an LED with broadoptical spectrum is used, e.g. 1.5 μm LED can be used as the lightsource 2. The output light from a light source 2 is split into two fiberbranches 6, 8, e.g., using a standard fused fiber splitter 4. One of theoutput arms 6 is selected for delivery of the light to themagnetic-field probe 20. The other half 8 is typically lost, but may beused for noise subtraction 10 in order to improve the noise performance.In the sensing arm 6, the light passes through the biasing unit 12 andreaches the magnetic field probe 20. The biasing unit 12 may include apolarizer 14 and a Faraday rotator 16. From the biasing unit 12 thelight travels along a further fiber and exits into the probe unit 20.The probe unit 20 incorporates a sensing polymer film 22 with a highVerdet value and optionally a mirror 24. Optionally a lens 26 can beprovided to shape the light beam onto the sensing film 22. The lightpasses through the sensing film 22 and is optionally reflected by themirror 24. The light may be incident at 90° to the polymer film or itmay be incident at an angle thereto. A measurement principle ofembodiments of the present invention is transmission through a filmmaterial with a high Verdet constant that is magneto-optically activeand sufficiently transparent. After passing through the sensing film 22twice and reflecting off the mirror 24, light with rotated polarizationtravels through the same fiber 18 as on the way towards the probe 20.After passing through the fiber splitter 4, the intensity of light isdetected using a photodetector 30 with associated read-out electronics(not shown). The polarization rotation in the magnetic-field probe 20 isconverted into intensity variation by the polarizer which isincorporated in the biasing unit 12. An additional (and very important)purpose of the biasing unit 12 is to create the phase bias necessary fordetection. Without the bias, the response of this system to polarizationrotation would be proportional to the cosine function of the rotationangle. Since Faraday rotation is in generally very small, a cosinefunction-type response will result in even smaller variations of thedetected intensity (˜ rotation angle squared). The passive bias adds alarge fixed angle to the Faraday rotation. This large angle typicallyequals 90 degrees, but may be chosen to be different. If the bias angleis 90 degrees, the cosine response is converted to the sine response,which is proportional to the polarization rotation angle, suitable fordetecting small rotation angles.

Accordingly, the present invention is based on detection of rotation ofthe polarization of light after passing through a sensing material,e.g., film, under the influence of a magnetic field. The sensingmaterial in the sensing film is affected by the magnetic field. Ameasurement principle of embodiments of the present invention istransmission through a film material with a high Verdet constant, i.e.the sensing material has a high Verdet constant. The sensing material ispreferably in the form of a film, i.e., its thickness in the directionof the optical path is less than its size in a direction perpendicularto the optical path. The thickness of the sensing polymer film willinfluence the resolution of the device. The thickness of the materialand the Verdet constant will also affect the transparency of the sensingfilm. Preferably, the thickness of the film is less than 1 cm, morepreferably less than 1 mm and most preferably of the order of 10microns. In embodiments of the present invention, the sensing materialcan be a conjugated polymer or a blend of a polymer withsuperparamagnetic nanoparticles. These materials show a very highFaraday rotation, indicated by their high Verdet constant (e.g.,>10⁶°/Tm) while still maintaining some transparency.

Measurement results are shown in FIG. 3 for a nanoparticle/polymer blendaccording to an embodiment of the present invention. The lower line ofround dots in the figure is a measurement series without such material,while the upper inclined line of square dots is a measurement serieswith a nanoparticle/polymer blend according to an embodiment of thepresent invention. As one can see the latter is extremely sensitive to achange in magnetic field. Since interferometers such as a fiber Sagnacinterferometer is able to detect rotations of microdegrees, one can seethat the sensitivity in terms of magnetic fields is very high. In fact,depending on the materials used, sensitivities on the order of 10⁻¹⁵ Tcan be achieved.

The optical circuit can be built using telecom-grade fiber-opticcomponents. If known inorganic materials with high Verdet constant areused for the sensing film, the resolution of the device is down to 10nanoTesla, which makes it useable for various robotic and monitoringapplications. As such the sensor can be used as a low-cost monitoringdevice for surge detection in high-voltage electric power lines. Noisereduction schemes and material with high Verdet constants cansubstantially improve the sensitivity, down to 10⁻¹⁵ T.

A preferred embodiment of the present invention comprises a sensor asfollows. The sensor utilizes the in-line fiber-based Sagnacinterferometer that is schematically shown in the FIG. 2. The majorcomponents of the interferometer are the light source 2 (fiber-pigtailedLED), a fully spliced fiber-optic circuit, a photodetector 20 anddetection electronics. The sensor is powered by a standard +/−12Velectric power supply or a battery. The magnetic-field probe 20 can bebased on inorganic materials, e.g., a stack of 500 μm thick BIG crystalsor more preferably on the organic sensing film materials of embodimentsof the present invention. The voltage output of the device isproportional to the magnetic field at the location of the probe 20. Twoelectrical outputs are available: DC- and AC-coupled. The AC-coupledoutput has an additional 50× gain. The sensor can operate at frequenciesup to 10 kHz (DC or AC coupled), and has a demonstrated sensitivity ofless than 10 nT, e.g., down to 10 femtoT. A much higher sensitivity canbe achieved by replacing the BIG-based magnetic probe with that based onan organic magneto-optical organic material such as a magneto-opticpolymer or polymer/nanoparticle composite in accordance with embodimentsof the present invention.

A two-fiber embodiment is shown schematically in FIG. 4. The light thattravels in a first fiber from the light source 2 is directed though abeam splitter 4, a polarizer 14 to a sensing film made of high-Verdetmaterial such as high-Verdet polymer or a polymer/nanoparticle compositewhich is located in the sensor head 20 as described with reference toFIG. 2. The measurement principle of embodiments of the presentinvention is transmission through a film material with a high Verdetconstant. After passing through the sensing film twice and beingreflected off a mirror, the light is coupled into a second output fiber.Preferably, polarization preservation is maintained in both input andoutput fibers. The returning signal is sent by the splitter 4 to adetector 30. A phase modulator 28 can be used. The polarization rotationin the magnetic-field probe 20 is converted into intensity variation bythe polarizer 14. As described for previous embodiments the thickness ofthe sensing polymer film will influence the resolution of the device andthe transparency of the film.

The interferometer system shown in FIG. 2 or 4 measures thenon-reciprocal phase shift that is accumulated as the lightwave travelsthough the fiber-optic circuit. The sensor detects the non-reciprocalphase shift due to the Faraday rotation in the sensing element, byconverting this shift into polarization rotation. When thisnon-reciprocal shift is due to Faraday rotation in the polymer probe(passing twice), it is given by the formula (see FIG. 8 passing once):Δφ=4VLBwhere V is the Verdet constant of the polymer material, L is the lengthof the magnetic field probe, and B is the strength of the field to bemeasured. The non-reciprocal phase shift is proportional to the externalmagnetic field. The relation is given by the formula on page 8.Naturally, the higher the Verdet constant of the field probe is, theweaker the magnetic field the sensor is capable of detecting. Theminimum value of the field that the sensor is capable of detecting isthus proportional to the minimum detectable non-reciprocal phase shift.This minimum phase is typically limited by the shot noise at thereceiver and is of the order of 10⁻⁸ radians per square-root Hz. Variousknown schemes have been developed in order to push this sensitivitylimit below 10⁻⁸ radians per square-root Hz, if required by application;these techniques typically rely on subtraction of the source intensitynoise from the signal. At the 10⁻⁸ phase sensitivity level, theestimated sensitivity of the sensor to magnetic field is of the order of100 femto-Tesla per square-root Hz or lower, assuming that themagnetic-field probe is up to 2 cm-long and the Verdet constant of thepolymer is 10⁶°/Tm or higher, e.g., 10⁷°/Tm. Even higher sensitivity canbe achieved by improving the phase resolution of the interferometer byknown noise-subtraction techniques such as described in P. Polynkin, J.de Arruda, J. Blake, “All-optical noise subtraction scheme for fiberoptic gyroscope”, Optics Letters, vol. 25, p. 147 (2000), or by usinglonger magnetic-field probe.

Note that the Sagnac interferometer itself detects the phase shift, notmagnetic field. The minimum shift that can be detected is limited by theshot noise performance, and the detection bandwidth. The shot noisedepends on the total optical power that reaches the detector. With ˜1 mWof optical power and detection bandwidth of 1 Hz, the shot noise limitsthe minimal detectable phase by ˜10⁻⁸ radians.

Sagnac interferometer sensors according to embodiments of the presentinvention can make used of a passive DC-phase bias that is introduced bymeans of an integrated fiber-pigtailed Faraday rotator 16. Compared tothe conventional phase modulation-demodulation technique used forintroducing the bias, this approach simplifies the optical circuit andthe signal processing electronics, and thus reduces the fabricationcosts substantially. Embodiments of the present invention can routinelymeasure magnetic fields of the order of 10 microgauss using a Bi:YIGgarnet probe, e.g., up to 500 mm-thick. Another desirable feature of thesensor is that the electronic part is based on low-power components.

An embodiment of the present invention relates to an integratedmagnetic-field probe based on a magneto-optical organic film such as amagneto-optical polymer film made from e.g. a magneto-optic polymer or apolymer/nanoparticle composite. Compared to BIG and YIG films, thepolymer-based probes have a potential for orders-of-magnitude highersensitivity and can be more economical. In addition, the polymer-basedmaterials as described in embodiments of the present invention can beprocessed into thin films and integrated into waveguide-typemagnetic-field probes, providing for high levels of system integration

Particular embodiments of the magnetic-field probe 20 are shown in FIGS.5 and 6. The embodiment of FIG. 5 a can be integrated with the in-linefiber Sagnac interferometer that is shown in FIG. 2 and repeated in FIG.5 b. In this embodiment, a fiber-pigtailed collimator 21 is followed bythe sensing polymer film 22 on an appropriate substrate, and then by amirror 24. In particular, the sensing film 22 can be directly attachedto or deposited on the mirror 24. The entire probe can be compactlypackaged into a compact housing 25, using modern optical packagingtechniques, e.g., used for packaging fiber-pigtailed fiber isolators.

FIG. 6 shows a part of a probe that uses an integration of two singlemode fibers 42, 44 and an analyzer/polarizer 46 a and 46 b and anintegrated sensing film 48. Two fibers 42, 44 have a core and cladding.At the end of each fiber 42, 44 a portion of the cladding is removed sothat two windows are formed in the fibers that face each other. The endsof the fibers 42, 44 are angled polished at 45° and coated with asuitably reflecting material 50, e.g., a metal layer, to form inclinedmirrors. Light entering from one fiber 42 is reflected off theangle-polished fiber tip and passes through the analyzer/sensingfilm/polarizer stack 46 a, b, 48, and is further reflected off theangle-polished fiber tip of the second fiber 44 to continue on thereturn path.

The present invention also includes within its scope a free-space Sagnacinterferometer including a magnetic probe in accordance with embodimentsof the present invention. A free-space Sagnac interferometer usesmirrors to direct the light through a gas such as air or a vacuum. Thefiber interferometer embodiments may have a somewhat lower sensitivity(by approximately an order of magnitude), but they can be made much morerobust, compact and inexpensive. A difference between the free-spaceSagnac and a fiber Sagnac sensors can be in the type of a light sourcethat is used: Free-space sensor can utilize a narrow-line laser source,while a broadband LED source is typically used in the fiber version.

Compared with the free-space Sagnac, the fiber Sagnac according toembodiments of the present invention is much more compact and rugged. Inprinciple, it is possible to construct a free-space Sagnac withsensitivity exceeding that of a fiber Sagnac by about an order ofmagnitude, but the free-space Sagnac will be orders of magnitude moreexpensive. Thus using the magnetic probes based on the high-Verdetconstant materials is a much simpler and more effective way of improvingthe bottom-line detection limit of the sensor.

In accordance with embodiments of the present invention,magneto-optically active inorganic and/or organic materials areprovided. In accordance with some embodiments of the present invention,these materials can be formed into films which transmit light and have ahigh Verdet constant. One type of material is a polymer matrix loadedwith superparamagentic nanoparticles selected so that the materialtransmits light. Another type of material is based on conjugate polymerswhich can be loaded with superparamagentic nanoparticles.

Faraday rotation is the rotation of the plane of polarization oflinearly polarized light due to magnetic field induced birefringence.The effect has been known for more than a hundred years and is commonlyused in optical isolators, for remote sensing of magnetic fields or inmagneto-optical imaging. Faraday rotation is at its strongest ininorganic substances containing paramagnetic ions or insuperparamagnetic and magnetic materials. For example, Tb Ga garnets areamong the most efficient materials known in terms of Faraday rotation.Unfortunately, these materials are often very expensive and difficult toprocess, they do not allow for miniaturization, or are not suitable forapplications at ambient temperatures. The present invention makes use inseveral embodiments of organic, e.g., polymeric materials for efficientFaraday rotation. A significant advantage of organic materials is theirease of processing, limited weight and the fact that they can becustom-tailored to meet specific requirements. Embodiments of thepresent invention include conjugated polymers and nanoparticle/polymerblends that have very high Faraday rotation, comparable or even muchhigher than inorganic materials.

Example 1 Conjugate Polymers

In the first example, the sensing material used is a conjugated polymer,e.g., is a conjugated polymer which can be loaded with superparamagenticnanoparticles. The amount of nanoparticles in the polymer can range from0.1 to 50 wt %, e.g. 1-10 wt %.

Examples of such polymers are shown below:

with R as any possible chemical group. However, other conjugatedpolymers such as poly(acetylene)s, poly(pyrrole)s, poly(aniline)s,poly(fluorene)s, poly(thiophene)s, polytetrathiafulvalenes,polynaphthalenes, poly(p-phenylene sulphide), poly(para-phenylenevinylene)s can be used.

Some experimental values of the Verdet constant of a polyalkoxythiopheneare given in the Table below. The Verdet constant was measured at threedifferent wavelengths, i.e., 633 nm, 830 nm and 1550 nm.

633 nm 830 nm 1550 nm polythiophene −2500000°/Tm −2500000°/Tm−1000000°/Tm

Examples of the synthesis of some conjugated polymers for use with thepresent invention are shown in the schemes below.

Embodiments of the present invention include regio-regularpolythiophenes. Materials with superior Tc-conjugation, electricalcommunication, and solid state morphology can be prepared by usingregiospecific chemical coupling methods that produce >95% 2,5′-couplingsof poly(3-alkylthiophenes) (P3AT). The McCullough method synthesizesHT(Head to Tail)-poly(3-alkylthiophenes) (PATs) at or near about 100%couplings. As illustrated below, this method regiospecifically generates2-bromo-5-(bromomagnesio)-3-alkylthiophene from a monomer, which ispolymerized with catalytic amounts of Ni(dppp)Cl₂(1,3-diphenylphosphinopropane nickel(II) chloride) using Kumadacross-coupling methods. The McCullough method can be illustrated asfollows:

The Rieke method differs from the McCullough method primarily in thesynthesis of the asymmetric organometallic intermediate. As illustratedbelow, a 2,5-dibromo-3-alkylthiophene is added to a solution of highlyreactive “Rieke zinc” (Zn*) to form a mixture of the isomers,2-bromo-3-alkyl-5-(bromozincio) thiophene and2-(bromozincio)-3-alkyl-5-bromothiophene. The addition ofNi(dppe)Cl.sub.2 (1,2-bis(diphenylphosphino)ethane nickel(II) chloride),a nickel cross-coupling catalyst, leads to the formation of aregioregular HT-PATs. The Rieke method can be illustrated as follows:

A more practical preparative synthesis of a regio-regularpoly(3-substituted thiophene) with alkyl substituents developed byMcCullough was effected by condensation of an in situ preparedmono-Grignard (selectively at the 5-position) species of2-bromo-3-alkylthiophene followed by nickel cross coupling by means of anickel catalyst. U.S. Pat. No. 6,166,172 discloses a method of formingpoly-(3-substituted) thiophene, comprising: providing a solublethiophene having at least two leaving groups; dissolving the solublethiophene in a solvent to form a mixture; adding an organomagnesiumreagent to the mixture; heating the solution to reflux to form asolution containing a regiochemical isomer intermediate; adding a Ni(II)catalyst to the solution; agitating the solution; and recovering thepoly-(3-substituted) thiophene.

Typical syntheses according to the McCullough and ‘modified” McCulloughmethodologies are the GRIM methodologies are given below:

McCullough and ‘modified” McCullough methodologies:

R₁ and R₂ can be: alkyl, hetero-atom containing alkyl, aryl, substitutedaryl, branched alkyl.

Freshly prepared lithium diisopropykamide (LDA) (1.1-2 mmol) is added toa solution of monomer (1.20 mmol) in dry tetahydrofuran (THF) (3 mL) at−78° C. After stirring for 30 min at room temperature, the solution isadded to a suspension of magnesium bromide or zinc chloride (1.80 mmol)in dry THF (4 mL) and stirred for another 30 min. Then the reactionmixture is transferred by means of a syringe into a suspension ofNi(dpp)Cl₂ in dry THF (7.5 mL). After stirring for 2 h, thepolymerization is quenched with HCl (1M in methanol) and the polymer isprecipitated into methanol filtered off and dried.

GRIM Methodologies:

R₁ and R₂ is typically: alkyl, hetero-atom containing alkyl, aryl,substituted aryl, branched alkyl.

A solution of monomer (0.50 mmol) in dry THF (2.5 mL) is cooled to 0° C.and RMgX₃, e.g. iPrMgCl₃ (0.50 mmol) is added. The reaction mixture isstirred for 1 h at 0° C. and then transferred by means of a syringe to asuspension of Ni(dpp)Cl₂ in dry THF (7.5 mL). After stirring for 2 h,the polymerization is quenched with HCl (1M in methanol) and the polymeris precipitated into methanol filtered off and dried.

Regioselective synthesis of a poly(3-alkylthiophenes) (P3AT) thatcontains almost exclusively head-to-tail couplings gives polymersexhibiting improved electrical conductivities and a greater effectiveconjugation length. The introduction of alkyl groups longer than butylto the 3-position of the thiophene unit yields moderate to highmolecular weight materials soluble in common organic solvents.Regio-regular poly(3-substituted thiophenes) with alkyl, aryl, andalkyl/aryl substituents are soluble in common organic solvents. Thelength of the alkyl side group affects melting point, conductivity etc.

The “defect-free” conjugation in these polymer chains leads to betterπ-π between-chain overlap and give rise to highly ordered, conductingpolymer structures in solid state films. This solid state structuralorder allows charges to travel freely without being trapped or retardedby defects. Therefore, regioregular HT-PAT films have much higherconductivity than their regiorandom analogs. In fact, HT-PATs representone of the classes of polymers with the highest electrical conductivity.Furthermore, because of the homogeneity of their 2,5′-ring-to-ringcouplings, they exhibit high extinction coefficients for the absorptionof visible light corresponding to the π-π* absorption for thesematerials. This absorption determines the quality of the conducting bandstructure which may be utilized when a regioregular poly(3-substitutedthiophene) with alkyl, aryl, or alkyl/aryl substituents is, upon doping,used as in applications requiring highly conductive thin films. Anotherbenefit of the regio-regularity of these materials is that they canself-assemble in the solid state and form well-ordered structures. Thesestructures tend to juxtapose thiophene rings systems through at-stacking motif and allow for improved inter-chain charge transportthrough this bonding arrangement between separate polymers, enhancingthe conductive properties when compared to regio-random polymers.

Regio-regular poly(3-substitutedthiophenes) with alkyl, aryl, andalkyl/aryl substituents and an oxygen atom in the β-position of the3-substituent, i.e. poly(3-alkoxythiophenes), have also been prepared bythe McCullough (nickel cross-coupling of2-bromo-5-magnesiobromo-3-alkoxythiophene) and GRIM (Grignard metathesisof a 2,5-dibromo-3-alkoxythiophene followed by a nickel cross-couplingreaction) methods. Specific examples of this class of materials includepoly(3-substitutedthiophenes) with 2,5-dioxahexyl and 2,5,8-trioxanonylsubstituents. In some instances, the 3-substitutent may contain aheteroatom other than oxygen (e.g., “S”) in addition to an oxygen atomin the α-position. The amount of regio-regular 2,5′-couplings for thesematerials exceeds 95%.

Regio-regular poly(3-substitutedthiophenes) with alkyl, aryl, andalkyl/aryl substituents and an oxygen atom in the β-position of the3-substituent demonstrate the same enhanced performance as otherregion-regular poly(3-substituted) thiophenes with alkyl, aryl, andalkyl/aryl with regard to processability, defect-free electronicstructure, high extinction coefficient, high electronic mobility,improved solid-state morphology. Furthermore, the heteroatomfunctionality increases the solubility of this material in polar organicsolvents like tetrahydrofuran, dimethylformamide (DMF),1-methoxy-2-propanol acetate (PMA), and chloroform, which enhances theprocessing options for this material and allowing the selectivedissolution of this layer in the presence of other device components.

The presence of impurities, notably metal ions, in this material mayhave serious deleterious effects on the performance ofpolythiophene-containing devices. There are several methods by whichimpurities may be removed from a conjugated polymer. Most of these arefacilitated by the ability to dissolve the polymer in common organic andpolar solvents. The solubility of these materials facilitates theirpurification such that they can be made suitable for their applicationupon doping.

Synthetic methods, doping, and polymer characterization, includingregioregular polythiophenes with heteroatoms in the side groups, isprovided in, for example, U.S. Pat. No. 6,602,974 and U.S. Pat. No.6,166,172. Additional description can be found in the article, “TheChemistry of Conducting Polythiophenes,” by Richard D. McCullough, Adv.Mat. 10(2), pages 93-116, and references cited therein. In particular,section 4 describes “Chemical Synthesis of Heteroatomic FunctionalizedSubstituents on PTs: Recognition Sites for Self-Assembly and ChemicalSensing.” Another reference which one skilled in the art can use is theHandbook of Conducting Polymers, 2nd Ed., 1998, Chapter 9, by McCulloughet al., “Regioregular, Head-to-Tail Coupled Poly(3-alkylthiophene) andits Derivatives,” pages 225-258, which is hereby incorporated byreference in its entirety. In particular, section VI describesheteroatomic substituents on HT PATs including ether groups andthioalkyl groups.

U.S. Pat. No. 6,602,974 describes conducting regioregular polythiopheneblock copolymers, including diblock and triblock types, which include asnon-thiophene component, for example, polyolefins such aspoly(meth)acrylates, polystyrene, and polymethylmethacrylates, andpolyurethane elastomers.

Further synthesis examples can be taken from: Regioregularity inPoly(3-alkoxythiophenes): Effects on the Faraday Rotation andpolymerization mechanism by G. Koeckelberghs et al., MacromolecularRapid Communications 27(22), 1920-1925 (2006).

Example 2 Superparamagnetic Nanoparticles

In another example, the sensing element is a polymer doped withsuperoparamagnetic nanoparticles. The polymer may be a conjugate polymeras disclosed above. However the present invention is not limited theretobut includes the use of transparent polymers loaded withsuperparamagentic nanoparticles. The polymer forming then matrix can beany suitable transparent polymer such as PMMA, polyethylene,polypropylene, polycarbonate, ABS, PVC, PVA, polyimide, polystyrene,etc., or a conjugate polymer as described with reference to thisinvention. The amount of nanoparticles in the polymer can range from 0.1to 50 wt %, e.g., 1-10 wt % or 1 to 20 wt %.

The material can be formed into a film. The film can be made by anysuitable method, e.g., compression molding, extrusion, spin coating,doctor blading, powder coating, etc. The material in film form willtransmit light. In order to make the film translucent or transparent itis preferable if the particles have a dimension smaller than thewavelength of the light used with the sensor. The shape of thenanoparticles can be cylindrical, spherical, triangular or any othershape with dimensions of the order of 1 to 1000 nm. The nanoparticlescan be iron oxides, Cobalt, Nikkei, cobalt ferrites on any other systemthat exhibits superparamagnetic behaviour. The amount of nanoparticlesin the polymer can range from 0.1 to 50 wt %, e.g., 1-10 wt %. Thepolymer matrix is typically polymethylmethcrylate (PMMA), but can be anysuitable polymer, including conducting polymers, that is able todisperse the nanoparticles.

In order to increase the Verdet constant, the polymer/nanoparticleblends can be poled by an external magnetic field. For this, the polymerwith nanoparticles is heated close to its glass transition temperatureand placed in an external magnetic field. The nanoparticles will nowalign according to the field lines of the magnet. Upon cooling down inthe presence of the field, the orientation of the nanoparticles isfrozen in. As a result of the poling, the Verdet constant of thematerial is improved, as well the transparency.

The particles may also be aligned by other means, e.g., mechanicalmeans. Rod-shaped particles can be used, and incorporated into a plasticmatrix. This material can then be extruded into film and optionallystretched in order to align the particles.

The material forming the particles may optionally be doped. Suitabledoping materials are transition metals, rare earths especiallylanthanoids, for example doping with dysprosium, gadolinium, terbium,erbium, etc. The amount of doping is typically less than 10 wt %, e.g.,6 wt %, 5 wt %, 4 wt %, 3 wt %, 2 wt % or 1 wt %.

Experimental results are shown below. The sample was a spincoated filmof a blend of ironoxide nanoparticles with polyisbutylmethacrylate on aglass substrate. Poling was done in two ways: poling in the plane of thesubstrate (parallel poling) or perpendicular to the substrate. TheVerdet constant of the unpoled film was 3.4 10⁵°/Tnn. Poling increasedthe Verdet constant by a factor of 2 in when poling was done in theplane of the film and by a factor of 10 when poling was doneperpendicular to the film plane.

parallel or perpendicular Not poled poled Fe₃O₄ nanoparticles, 5 3.410⁵°/Tm 3.6 10⁶°/Tm wt % in 7.4 10⁷°/Tm polyisobuthylmethacrylate

To obtain the highest Verdet constants it may be necessary to combineaspects of the present invention, e.g., to use doped superparamagneticparticles in a polymer matrix, and to pole and align the particles. Itmay also be necessary to use a conjugate polymer as the polymer matrixor as a component of the polymer matrix.

If this is done the Verdet constant of nanoparticle based materials in apolymer matrix can be made in the range: 10⁶ to 10⁹° Tim, e.g., 10⁶ to10⁸°/Tm.

A typical synthesis of the nanoparticles is as follows:

Fe (III) chloride was dissolved in ethylene glycol in the presence ofoctylamine that acts as a stabilizer. The mixture was heated to 180° C.,and kept at that temperature for 24 h. The nanoparticles of Fe₃O₄ thatwere formed were purified by magnetic filtration. The size of theresulting nanoparticles is typically between 9 and 40 nm, depending onthe stabilizer used.

Cobalt ferrite nanoparticles can be synthesized by the same procedure,but in that case a small amount of cobalt chloride was added to thereaction mixture.

A synthesis of superparamagnetic magnetic nanoparticles is given below:Add 37.5 mL ethylene glycol and 25 mL octylamine in a 100 mLround-bottomed flask with 2 or 3 necks. Heat the mixture with constantstirring up to 150° C. Dissolve 2.4 g of iron(III) chloride in 10 mLethylene glycol and 4 mL distilled water. Add the dissolved iron(III)chloride dropwise into the hot octylamine-ethylene glycol mixture. Keepstirring the mixture and heat further until reflux (180-190° C.) for18-24 hours. Finally purify the particles by magnetic filtration.

The octylamine and ethylene glycol may combine to form a surfactantwhich stabilizes the superparamagnetic magnetite nanoparticles.

The shape of the superparamagnetic nanoparticles can be cylindrical,spherical, triangular or any other shape with dimensions of the order of1 to 1000 nm. The nanoparticles can be iron oxides, cobalt, nickelcobalt ferrites or any other system that exhibits superparamagneticbehaviour. Yu. A. Barnakov et al. in 2004 in Journal of Physics andChemistry of Solids Volume 65, pages 1005-1010 reported that Faradayrotation spectra measured on superparamagnetic nanoparticles in apoly(methylmethacrylate matrix) exhibited a broad spectral band centredat 650 nm (1.91 eV) for 200 nm-sized nanoparticles. Decreasing theparticle size leads to a significant narrowing of this band and theappearance of an additional peak in the 2.5-3.2 eV range.

Alternatively superparamagnetic magnetite nanoparticles can be preparedby removing oxygen from a 3-necked flask with nitrogen gas inside aglove box. Mix iron(III) chloride, iron(II)chloride tetrahydrate,nanopure water and ammonium hydroxide and heat to give a murky solution.Settle the murky solution over a permanent magnet, remove the clearsupernatant liquid, wash and dry to give black superparamagneticmagnetite nanoparticles with a particle size of 7.88 nm. Free amine(—NH₂) groups encompass the magnetite nanoparticles. The exact nature ofthe bonding has not yet been clearly understood.

To illustrate the sensitivity of the polymeric materials, we havemeasured Faraday rotation, due to the magnetic field of the earth, in athin film of a polyalkoxythiophene. The results are shown in FIG. 7. Ascan be seen the Faraday rotation is sensitive to the direction betweenearth's magnetic field and the direction of light propagation (θ). Whenthe earth's magnetic field is parallel with the light propagation (θ=0),Faraday rotation is maximum. On the other hand, when the magnetic fieldis perpendicular to the propagation direction, Faraday rotation isminimum.

In the Table below a comparison is provided of the Verdet constant of TbGa garnets (inorganic) compared with that of conjugated polymer andnanoparticle/polymer blends according to embodiments of the presentinvention.

Tb Ga Conjugated Nanoparticle/ Garnet Iron garnets polymer polymer (BIG,YIG, . . .) (polythiophene) blend Verdet constant 5000-1000000 10⁶-10⁷10⁷-10⁹ (°/Tm)

Advantages of embodiments of the present invention comprise at least oneof the following:

1) Faraday rotation in inorganic materials has been used frequently tomeasure magnetic fields. However, the Verdet constant of inorganicmaterials is usually low (<10⁶°/Tm), or the material suffers from poortransparency. As a consequence, the sensitivity of the Faraday sensor islimited. Organic materials, or blends of inorganic nanoparticles withorganic polylmers as described with respect to embodiments of thepresent invention can have much higher Verdet constants (>10000°/Tm).Furthermore, they are often much more transparent, leading to a Faradaysensor that is much more sensitive.2) Embodiments of the present invention provide an economicalreplacement of expensive SQUID magnetometers. The magneto-opticalorganic materials used to form polymer-based devices according toembodiments of the present invention are much more robust, compact,inexpensive, and will not require cryogenic cooling.3) The magneto-optically active organic materials such as polymersaccording to embodiments of the present invention result in devices withthe sensitivity potentially exceeding that of SQUIDs.4) There are two main advantages of the passively biased sensor comparedto the lock-in based version:

Firstly, the detection bandwidth of the passively biased sensor islimited only by the time of flight of light through the optical circuit.Thus, in principle, the bandwidth can be as high as 100 MHz or evenhigher. In comparison, the detection bandwidth of the lock-in sensorcannot be higher than the modulation frequency. In fact, the bottom-linebandwidth is at least an order of magnitude lower than that.

Secondly, the passively biased version is much simpler and more low-costbecause it does not require an optical phase modulator and sophisticatedphase detection electronics.

REFERENCES

-   T. Verbiest, S. Foerier, M. Vangheluwe, A. Persoons, G.    Koeckleberghs, P. Gangopadhyay, SPIE proceedings 2006, 6331, Paper    34-   G. Koeckelberghs, M. Vangheluwe, K. Van Doorsselaere, E. Robijns, A.    Persoons, T. Verbiest, Regioregularity in poly(3-alkoxythiophene)s:    Effects on the Faraday rotation and polymerization mechanism,    Macromolecular Rapid Communications, 27, p. 1920-1925, 2006-   R. de Carvalho, J. Blake, and G. Sanders, “Sagnac interferometry for    measurements of true nonreciprocal effects,” in Proc. SPIE 2070,    1993, pp. 264-269.-   J. Blake, P. Tantaswadi, and R. T. de Carvalho, “In-line Sagnac    interferometer current sensor,” IEEE Trans. Power Delivery, vol. 11,    pp. 116-121, 1996.-   G. Frosio and R. Dandliker, “Reciprocal reflection interferometer    for a fiber-optic Faraday current sensor,” Appl. Opt., vol. 33, no.    25, pp. 6111-6122, 1994.

The invention claimed is:
 1. A sensor of magnetic fields for use with alight source, comprising: a beam splitter for splitting a light beaminto a first and second direction, a magnetic sensor probe in a magneticfield for receiving light in the first direction and for returning lightwith a rotated polarization in accordance with the magnetic field, and adetector for detecting the returning light to thereby provide a valuerelating to the magnitude of the magnetic field, whereby the magneticsensor probe comprises an inorganic magneto-optically active film, or anorganic magneto-optically active film, or both; wherein the filmcomprises a polymer that further includes superparamagnetic particles;wherein a Verdet constant of the film is increased by one or acombination of the following: poling, aligning and doping thesuperparamagnetic particles; and wherein the film is arranged totransmit the light received by the magnetic sensor probe.
 2. The sensorof claim 1, being a Sagnac interferometer.
 3. The sensor of claim 2,wherein the Sagnac interferometer is a fiber Sagnac interferometer. 4.The sensor of claim 1, wherein the film comprises a conjugated polymer.5. The sensor of claim 4, wherein the conjugated polymer is selectedfrom the following group: poly(acetylene), poly(pyrrole), poly(aniline),poly(fluorene), poly(thiophene), polytetrathiafulvalenes,polynaphthalenes, poly(phenylene sulphide), poly(para-phenylenevinylene), polyalkoxythiophene.
 6. The sensor of claim 4, wherein theconjugated polymer is:


7. The sensor of claim 1, wherein the superparamagnetic particles arenanoparticles having a dimension smaller than the wavelength of thelight from the light source.
 8. The sensor of claim 1, wherein thesuperparamagnetic particles are doped with a lanthanoid.
 9. The sensorof claim 1, wherein the superparamagnetic particles are iron oxides,cobalt, nickel cobalt ferrites.
 10. The sensor of claim 1, wherein theamount of superparamagnetic particles in the polymer ranges from 0.1 to50 wt %, or 1-10 wt %.
 11. The sensor of claim 1, wherein the inorganicmagneto-optically active film, or the organic magneto-optically activefilm, or both have a Verdet constant>10⁶°/Tm, or 10⁷°/Tm or higher, or10⁸°/Tm or higher or up to 10⁹°/Tm.
 12. A sensor of magnetic fields foruse with a light source, comprising: a beam splitter for splitting alight beam into a first and second direction, a magnetic sensor probe ina magnetic field for receiving light in the first direction and forreturning light with a rotated polarization in accordance with themagnetic field, and a detector for detecting the returning light tothereby provide a value relating to the magnitude of the magnetic field,whereby the magnetic sensor probe comprises an inorganicmagneto-optically active film, or an organic magneto-optically activefilm, or both; wherein the magnetic sensor probe comprises two opticalfibers, and the inorganic magneto-optically active film, or the organicmagneto-optically active film, or both are integrated into a reflectivetermination of the two optical fibers.
 13. A method of sensing amagnetic field, the method comprising: directing a light beam through amagnetic sensor probe in a magnetic field and receiving returning lightwith a rotated polarization in accordance with the magnetic field fromthe magnetic sensor probe; and detecting received light to therebyprovide a value relating to the magnitude of the magnetic field whereinthe directing a light beam through the magnetic sensor probe includes:directing the light through an inorganic magneto-optically active film,or an organic magneto-optically active film, or both; wherein the filmcomprises a polymer that further includes superparamagnetic particles;wherein a Verdet constant of the film is increased by one or acombination of the following: poling, aligning and doping thesuperparamagnetic particles; and arranging the film to transmit thelight received by the magnetic sensor probe.